Polymeric substrates are used in many modern day products, such as manufacturing, biomedical products, or even consumer products. Each application may require different properties of the polymeric substrate. For example, one product may benefit from a polymeric substrate that is very wear-resistant, while another product may benefit from a polymeric substrate that glides or slides along another object. Accordingly, it would be beneficial to have methods to provide polymeric substrates with the ideal properties for a particular application.
One way that a polymer surface can be modified is by polymer surface crosslinking. Polymeric substrates are modified to increase the extent of cross-linking between the polymer chains in the near-surface region of the polymer. Surface crosslinking (polymerization) may be done, for example, to increase the wear resistance of polymer surfaces used in various industrial and biomedical applications. Pristine polymer surfaces generally possess a weak surface layer consisting of uncrosslinked chains, which is detrimental to polymer wear resistance against other surfaces (Hansen, R. H. and Schonhom, H., Polym. Lett., 4:203 (1966); Egitto, F. D. and Matienzo, L. J., IBM J. Res. Develop., 38:423 (1994)). The replacement of this weak layer by a crosslinked layer is beneficial to the cohesive strength of adhesive joints (Hansen, R. H. and Schonhorn, H., Polym. Lett., 4:203 (1966)). The crosslinked layer can also provide a diffusion barrier against solvents and moisture that affect negatively the interfacial adhesion strength (Egitto, F. D. and Matienzo, L. J., IBM J. Res. Develop., 38:423 (1994)), which is critical to maintaining a strong bonding of polymers to other surfaces.
Direct energy transfer from energetic particles (i.e., ions and uncharged particles) and/or radiation (i.e., vacuum ultraviolet (VUV) and ultraviolet (UV) light, γ-ray, and X-ray) to the polymer surface induces surface crosslinking (Dong, H. and Bell, T., Surf. Coat. Technol., 111:29 (1999). Conventional γ-ray and X-ray treatments are bulk treatments that tend to degrade the mechanical properties of polymers, such as fracture toughness (Baker, D. A. et al., Polymer, 41:795 (2000)). Therefore, surface-specific methods resulting in polymer surface crosslinking while preserving the bulk properties are more effective than bulk treatments.
One surface-specific method is polymer surface crosslinking by an inert gas plasma (Hansen, R. H. and Schonhorn, H., Polym. Lett., 4:203 (1966)). The effect of plasma parameters (such as power of plasma source, distance of source from the polymer surface, pressure, flow rate of gas used to generate plasma) on the crosslinked layer thickness has been studied (Flory, P. J. and Rehner, J. Jr., J. Chem. Phys., 11:521 (1943)). For plasma-treated polyethylene, the thickness of the crosslinked layer is typically between 0.3 and 1.6 μm, depending on the plasma gas, power, and treatment time (Yao, Y. et al., J. Adhes. Sci. Technol., 7:63 (1993)). The general trend is for the crosslinked layer thickness to increase with the plasma power and the processing time.
Similarly, for ion implantation techniques, the ion dose is altered in order to vary the depth of implantation (and thus crosslinking). Shields have been used to protect the surfaces from any modification or to control the depth of implantation.
However, the increase in the thickness of the cross-linked layer beyond a certain value has a limited effect on the actual properties of the surface itself. Accordingly, new methods and systems are needed to provide polymeric surfaces with specific mechanical properties.
Additionally, to determine whether a product has the required mechanical properties, the properties need to be evaluated. The chemical bonding of crosslinked layers has been studied by electron spin resonance spectroscopy and X-ray photoelectron spectroscopy, and the crosslinked layer thickness has been measured by the swelling and rheological techniques. Although these methods are effective in identifying crosslinking and radicals on plasma-treated polymer surfaces, they do not provide information for the mechanical modification. Surface chemical changes and the presence of radicals cannot be directly correlated to the strength of the crosslinked surface. In fact, radicals may result in chain scission, which is detrimental to the mechanical strength of the polymer surface. (Momose, Y. et al., J. Vac. Sci. Technol. A 10:229 (1992))
Embodiments of the invention address the above problems of surface modification and measurement, and other problems, individually and collectively.